Fertilisation

FOREST SOILS

For many years (1920s -70s) government policy in Ireland was to confine forestry to lands that were marginal or unsuitable for agriculture. The result of this policy is that there are large areas of state forestry plantations established on wet, nutrient poor soils in exposed areas.  It is only in recent years (1980s to date), with the introduction of attractive grant aid for the establishment of forests, that forestry is being planted on more fertile land, mostly by farmers.

Sitka spruce showing nutrient deficiency (photo D. Tiernan)

Forests established on poor infertile sites need fertiliser if they are to develop into mature stands capable of producing commercial timber. Usually one application of fertiliser at planting is sufficient to establish the crop, but sometimes a second application is required to get the trees to the closed canopy stage. At that point nutrient cycling takes place (i.e. the release of nutrients from fallen needles and twigs) which is generally sufficient to maintain the trees to the end of the rotation.

Forests, even those growing on poor sites, require little input of fertiliser during a forty year rotation. Trees are able to draw on nutrients from the deeper layers of the soil horizon better than most other vegetation because of their extensive deep root systems.

FERTILISERS

Phosphate (P) is the most common nutrient missing in Irish forest soils. This is normally supplied in the form of rock phosphate which is mined in North Africa.  This is a naturally occurring fertiliser obtained from ancient sea deposits and ground into a fine powder which is then pelleted for ease of application. Rock phosphate is a slow release fertiliser which supplies the growing trees with this essential nutrient over several years.

Another element that is sometimes required is Potassium (K), particularly on midland peat soils where deficiencies can sometimes occur. In other parts of the country trees get sufficient potassium from rainfall as it is picked up by high winds over the sea and transported inland. Potassium is often applied in the form of muriate of potash.

Nitrogen (N) is also required for successful tree growth and this usually applied in the form of calcium ammonium nitrate.

Deficiencies in trace elements are generally not a problem in Irish forests but copper deficiency can sometimes occur in the peat soils of the midlands.

FERTILISER APPLICATION

First application

Depending on the fertility of the soil, fertiliser is applied to the site after planting. Standard practice is to apply ground rock phosphate as follows:

Enclosed/Improved fields recently farmed                None

Former agricultural land not recently worked           250 kgs/ha

Unenclosed land (bog or open mountain)                 350 kgs/ha

Fertiliser is applied by a tractor mounted spreader or by hand.  A minimum buffer zone of 20 metres is left unfertilised next to water courses (only 1.5 metres required for agriculture).

Second application

Second manuring by helicopter (photo D. Tiernan)

On some sites a second application is required when the plantation reaches 8-10 years of age. The amount and type of fertiliser needed is determined by foliar analysis. Shoots are collected during the dormant season and sent to a laboratory where the nutrient content of the crop is accurately determined. Specific recommendations on the type and quantity of  fertiliser to apply are made to the forest manager/grower. This process ensures that only sufficient amounts of fertiliser are applied to get the crop growing again and that over fertilisation does not occur, which is costly in both financial and environmental terms.

Access through the plantation can be difficult at second fertilisation stage and applications are usually made by helicopter. This operation is carried out by experienced helicopter crews assisted by state of the art GPS equipment to ensure accuracy and evenness of application and to avoid sensitive areas such as rivers, streams or wetlands.  Consultations with the Forest Service, Fishery Boards and local people are carried out prior to the operation taking place.

Aerial fertilisation usually takes place during dry weather in the months of April to August.  At that time of the year any of the available nutrients are absorbed by the growing trees and vegetation thus avoiding runoff.

Fertilisers, if not carefully applied have the potential to affect water quality. Nutrient run-off enriches water ways (eutrophication) causing algal blooms that reduce the oxygen content of the water. This in turn affects the insect, fish and animal life that live in or depend on the waters.  Many of the forests established on western peatlands can no longer be fertilised as nutrient run-off can affect streams and rivers which are important trout and salmon fisheries, or are habitat for the protected  fresh water pearl mussel.

Forestry and Water Guidelines on best practice have been developed by the Forest Service in consultation with the forestry sector, fishery boards and other stakeholders. Coillte have developed their own guidelines which go beyond these guidelines in order to comply with the requirements of forest certification.

NUTRIENT CYCLING

From the time trees are planted to the time they close canopy (i.e. when the branches of adjacent trees meet) nutrients are absorbed from the soil and are retained by the tree, mostly in the branches and foliage.  As the tree grows taller the lower branches die as a result of shading from those higher up the trunk. The needles and twigs (litter) are shed and decay on the forest floor releasing nutrients which are absorbed by tree roots. Nutrient cycling now takes place and the trees are able to maintain healthy growth through the supply of nutrients from both the litter and the soil.

Plantations established on infertile sites are usually able to maintain healthy growth for 8-10 years drawing on the nutrients supplied by the first application of fertiliser (usually rock phosphate). As the nutrient supply becomes reduced growth also reduces and often ceases. Plantations can then remain in a checked or moribund state for many years never forming closed forest stands. A second application of fertiliser is then needed to boost growth and get the trees to the closed canopy stage when nutrient cycling will maintain growth to the time the trees are harvestable.

NUTRIENTS AND PEATLAND FORESTS

Large area of forests on the western seaboard of Ireland, have been planted on peatland sites.  Many of these peatlands are inherently infertile and the application of nutrients is required to ensure the healthy growth and development of plantations on these sites. Peat, because of its organic nature, is unable to retain nutrients and these can be flushed out of the forest if they are not taken up by the trees or vegetation.

Nutrients released from brash (needles and branches) following clearfelling are highly mobile on peat sites and this can result in high concentrations entering in the watercourses, causing enrichment of waters and eventual pollution. Species such as the freshwater pearl mussel, which is protected under the EU Habitats Directive, is particularly sensitive to water pollution and special measures are taken when felling trees within catchments in which the mussel occur.  Best practice guidelines have been developed by the Forest Service, National Parks and Wildlife Service and Coillte  involving  the installation of  buffer zones,  limiting felling coupe sizes etc.

At the time the peatlands were planted there was little appreciation of the unique biodiversity value of many of these sites, nor of their role in controlling the flow of water in streams and rivers. This situation has changed and today most of these areas would not be planted; but now that they do exist they must be managed into the future. Restoration to their original pre-afforestation state is not possible in many instances as the sites have been irreversibly changed due to intensive drainage and fertilisation. Some sites, however, can be restored and Coillte has embarked on a restoration programme for sites that have been planted in what are now designated as Special Areas of Conservation ( SACs).

Low input forestry is now being practiced on many of the restocked sites in peatland areas. These are being re-planted with the lodgepole pine which is a low nutrient demanding species and fertilisers are not being applied. The pine is able to utilise the existing nutrients from the previous crop released from the decaying brash.

Further information at:

Fertiliser application to conifer plantations on oligotrophic peat sites